Sidney Thornton, a two-time Super Bowl champion running back who played his entire career with the Pittsburgh Steelers, has died, the team announced Wednesday. He was 68.

The cause of Thornton’s death was not announced.

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Sidney Thornton vs the Eagles

Running back Sidney Thornton, of the Pittsburgh Steelers, runs with the football against defensive lineman Carl Hairston, #78 of the Eagles, during a preseason game at Veterans Stadium on Aug. 15, 1981, in Philadelphia. (George Gojkovich/Getty Images)

"We are saddened by the loss of former Steelers RB Sidney Thornton," the Steelers said in a tweet.

Thornton starred at Northwestern State, where he set career records in rushing attempts (537), rushing touchdowns (25) and rushing yards (2,662) when he turned pro. Those marks have since been broken. Pittsburgh would choose Thornton in the second round of the 1977 NFL Draft with the 48th pick.

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Sidney Thornton in 1980

Sidney Thornton, of the Steelers, carries the ball against the Houston Oilers, Sept. 7, 1980, at Three Rivers Stadium in Pittsburgh. (Focus on Sport/Getty Images)

Thornton played six seasons with the Steelers from 1976 to 1982.

He played in 74 games for the Steelers and racked up 1,512 yards and 18 rushing touchdowns. He also had 46 catches for 515 yards and six touchdowns. He appeared in the playoffs three times with the Steelers and was a part of their Super Bowl-winning teams in 1978 and 1979.

Thornton was a backup behind Franco Harris and Rocky Bleier on those teams He had a breakout game against the Miami Dolphins in the divisional round of the 1979 playoffs. He ran for 52 yards and a touchdown in the 34-14 win. Pittsburgh would hoist the Vince Lombardi Trophy that year.

Sidney Thornton vs the Browns

Sidney Thornton, of the Pittsburgh Steelers, carries the ball against the Browns on Oct. 15, 1978, at Cleveland Municipal Stadium in Cleveland. (Focus on Sport/Getty Images)

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He took two years off from football before playing one season in the United States Football League with the Oklahoma Outlaws.